Reverse osmosis system

ABSTRACT

A reverse osmosis system comprises a permeate collection tube which is connected at one end to a distribution system for permeate which comprises at least one device for cleaning and/or disinfection. The permeate collection tube, the cleaning and/or disinfection device and a circulation pump are arranged in a circulation circuit.

The present invention refers to a device for water treatment accordingto the reverse osmosis principle. Devices of such types, reverse osmosissystems, are particularly used in combination with hemodialysis devicesto obtain sterile high-purity water from tap water for preparing thedialysis liquid.

The present invention particularly aims at a liquid that is as sterileas possible.

This object is achieved by a reverse osmosis system which can be coupledwith at least one consumer, particularly a dialysis device, to supplythe consumer with high-purity permeate, comprising a raw-water inletline which supplies raw water to a filter module the primary circuit ofwhich is separated by a semipermeable membrane from a secondary circuitwhich comprises a permeate collection tube which is connected at one endto a distribution system for permeate which comprises a permeate inletline with at least one connection to which a consumer can be coupled,and a permeate return line. The permeate distribution system comprisesat least one device for cleaning and/or disinfection. A concentrateoutlet line leads away from the primary circuit of the filter module.The permeate collection tube, the cleaning and/or disinfection deviceand a circulation pump are arranged in a circulation circuit. Furtherdetails and configurations of the invention become apparent from thefollowing description of embodiments taken in conjunction with thefigures, of which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the scheme of a typical reverse-osmosis system according tothe prior art;

FIG. 2 shows the scheme of a comparable reverse-osmosis system withequipment features according to the invention;

FIG. 2a shows an alternative embodiment;

FIGS. 3-4 show the scheme of associated devices.

DETAILED DESCRIPTION OF THE DRAWINGS

As is generally known, the functional principle of reverse osmosissystems consists in that the water to be treated is guided in a filtermodule under high pressure along the surface of a semipermeablemembrane, with part of the water, the so-called permeate, being guidedover the surface of the spirally wound membrane in such a manner that itexits through the membrane and is collected at the other side of themembrane within the module in a permeate collecting tube and is suppliedfrom there via hydraulic connections to the points of consumption.

The part of the raw water that does not pass through the membrane and isenriched with retained substances, the so-called concentrate, flows atthe end of the flow section out of the filter module.

The scheme shown in FIG. 1 illustrates, as a typical example, thecooperation of essential functional elements of a reverse osmosis systemaccording to the prior art. The raw water to be treated flows out of thefeeding line 1 and via the valve 4 into a buffer vessel 5 with installedfill level control 6. The water passes out of this container 5 throughthe line 9 via the pump 7 into the filter module 10, the primary circuit11 of which is separated by the semipermeable membrane 49 from thesecondary circuit 12. The permeate flows out of the secondary circuit 12into a ring line 23/24 from which the consumer lines 25 divert. Permeateproduced in excess can flow back at the end of the ring line 23/24 viaan inserted pressure-maintaining valve 26 into the vessel 5, the settingof said valve determining the pressure prevailing in the ring line23/24.

The pressure needed for filtration in the primary circuit 11 of thefilter module 10 is produced by the pump 7 in combination with a flowresistance means 16 which is inserted into the concentrate line 13downstream of the filter, e.g. in the form of a throttle valve or apressure valve.

Reverse osmoses particularly also serve the recovery of sterile water.

The part of the supplied tap water that does not pass through the filtermembrane 49 and is enriched with retained chemical water substances andwith bacteria forms a biofilm on the inner surfaces of theliquid-conducting system.

The deposits of the biofilm in the primary circuit 11 can pass throughthe non-ideal filter membrane 49 as pyrogens and endotoxins andcontaminate the high-purity permeate side 12/23/24.

These parts will deposit there as a biofilm and contaminate thepermeate.

For the purpose of simplification the term cleaning is chosenhereinafter for the description of the decontamination, disinfection andpurification measures.

One reason for the bacterial enrichment of the permeate must also beseen in the permeate collection tube area through which no or only amoderate flow is passing. This line section of the high-purity secondaryside can only be thermally or chemically disinfected and cleaned undergreat efforts.

Constructionally, the filter membrane of the reverse osmosis module isspirally wound around a permeate collection tube. Part of the liquid ishere supplied from the primary side to the secondary side through themembrane to the permeate collection tube and fed from said place to theconsumer.

The reason for the bacterial enrichment in the permeate collection tubeis on the one hand the restricted, relatively small transmembranic flowthrough the filter membrane to the permeate collection tube, and on theother hand the cleaning process for the permeate collection tube alwayspresupposes a transmembranic cleaning operation, i.e. a cleaning of theprimary side, and thus of the total system, and this operation is cost-and time-intensive.

However even if the secondary-side distribution system is cleaned, thereis the problem that one cannot guarantee absolute sterility, especiallyon the basis of normative guidelines, because a re-contamination fromthe permeate collection tube is very likely.

Another reason for re-contamination is that with the known applicationsthe bacterial growth within the secondary-side distribution system canonly be prevented up to the transfer point to the consumer by means ofcleaning measures.

For instance in dialysis devices with a free inlet according to EN 1717one does not take into account the path leading from the transfer pointof the ultrapure water line of the reverse osmosis system up to the freeinlet of the dialysis device.

Likewise, when permeate is stored e.g. in ultrapure water tanks or bags,the filling line is not fully included in the disinfecting process. Thisproblem is also applicable to mixing facilities for producing medicinalsolutions on the one hand from powdery, paste-like, granulated or otherhighly concentrated raw materials and on the other hand from high-puritypermeate on condition that these systems also contain a permeate storagemeans with feed lines for the permeate or the permeate production.

To keep this route sterile, considerable resources in the form ofelectrical energy during hot cleaning or also of chemical disinfectantsand of the manpower hired would be needed because it is normally notonly the secondary distribution system, but also the RO system and theconnected consumer, e.g. the dialysis device, that have to be integratedinto the disinfecting process.

Apart from the contamination of the waste waters, especially the inputof large amounts of chemical disinfectants poses considerable safetyrisks for patients because the slightest amounts or residues in thedevice or in the conduits may have toxic impacts on patients. Suchimpacts can only be prevented by way of large flushing volumes and acareful control as to residues.

As a rule, the disinfecting measures are therefore limited to therespectively connected devices; the interfaces and the ultrapure watertransfer points, respectively, are not taken into consideration.

Exceptions are here integrated hot-cleaning operations in the case ofwhich great amounts of hot water are prepared and e.g. supplied todialysis devices and conveyed therethrough to the outlet.

The aim is to ensure absolute sterility of the high-purity distributorcircuit including the connected consumer line and, nevertheless, toreduce the operating costs for hygienic measures.

A fully automatic purification is here needed without any risk forpatient and user and without carrying out, as is presently superfluouslydone, the disinfection of the reverse osmosis system and the connectedconsumers with chemicals in an imperative way.

It should here be the objective to achieve the lowest possible energyinput and no negative impact on the waste waters by chemicals or thermalenergy.

The aim should be a partial disinfection of the whole high-puritydistribution system, i.e. the secondary side, also within the connectedconsumer in dialysis devices up to the water inlet, i.e. the supplytank. Different cleaning methods at the primary and secondary side ofthe membrane can be carried out. A complete integrated cleaning of thewhole system is here also possible.

A measuring option for measuring the extent of the soiling/contaminationdegree and a resulting warning or the automatic introduction of acleaning operation, respectively, are needed.

Control as to the absence of disinfectants shall not be required.

Gentle cleaning methods should be chosen to prevent any restriction ofthe service lives of the components used, particularly the filtermembrane.

According to the invention the permeate collection tube of the membraneis included in the cleaning of the high-purity secondary permeatedistribution.

The module of the reverse osmosis system is here operated preferably asa 4-pole module, resulting in a closed secondary circuit with minimaldead spaces that is to be cleaned independently of the primary circuitof the reverse osmosis system.

With great advantage a further pump is used for this in the secondarycircuit. Primary side and secondary side have thereby to be cleanedindependently of one another by means of different cleaning methods andintervals.

With advantage the invention provides a cleaning method in whichozonization of the secondary distribution system takes place. This canalso take place in alternation with a hot cleaning of the reverseosmosis system or also of the total system. Chemical cleaning or acombination of thermal and chemical cleaning is here not explicitlyruled out.

It has also been found that the combination of least dosed ozone andincreased temperature in the water only causes minimal damage tomaterial because oxidation and decomposition processes of the ozone areaccelerated. As a consequence, this method also permits an ozonizationof the reverse-osmosis primary side and thus of the membrane.

Likewise, an ozone input into a possibly connected storage containere.g. in a mixing system in communication with the reverse osmosissystem, makes sense in order to keep the permeate-prepared solutionmicrobiologically stable.

With great advantage, apart from the water inlet valve of the consumer,a further valve is connected to the outlet particularly in dialysisdevices. This further valve passes the cleaning agent during cleaning upto the inlet valve of the connected consumer. Without the operation ofthe consumer the interface from the distribution system of the reverseosmosis system to the dialysis-device water inlet can thereby be cleanedalmost without any dead spaces.

With advantage the invention provides for a cleaning chamber in theprimary circulation circuit of the reverse osmosis system, the design ofwhich allows and provides for an electrical or magnetic orelectromagnetic or electrolytic or sonographic effect or a combinationof different physical effects of the liquid flowing therethrough.

It is the function of the cleaning chamber to prevent, on the one hand,any decontamination by the microorganisms and on the other hand thestabilization of the hardeners, so that deposits on the reverse osmosismembrane are prevented.

Use and place of installation of the cleaning chamber are however notrestricted to the described function.

Since the disinfection action of the electrolytically produced oxygenradicals as well as the stabilization of the lime crystals in the liquidare only temporary after the cleaning chamber has been switched off, thehigh-pressure throttle is advantageously opened periodically and/or atthe end of an operating cycle either by motor or, if a fixedflow-resistance means is installed, by means of a bypass valve withdischarge valve. This suddenly increases the flow in the primarycirculation circuit and the surfaces of the liquid-conducting componentsare flooded and flushed.

Since the effect of the cleaning chamber cannot be determined by theuser directly through its physical effect or its effects on crystalformation, a cleaning sensor is provided with great advantage for theprimary circuit.

Components or liquid-conducting lines can here be configured withtransparent or translucent material to check the contamination visuallyor opto-electronically.

In an advantageous configuration the associated transmitter/receiverunit is arranged in one plane. The optical transmitter signal is hereprojected onto an opposite reflecting surface and is reflected fromthere to the optical receiver.

The preferred solution is a transparent tube section with oppositetransmitter/receiver sensors. The quantity of the receiver signal ishere a direct function of the degree of soiling.

With great advantage, in order to improve the impact in time and toenhance the physical cleaning effects, an additional circulation pumpcan be connected with a cleaning chamber between the concentrate outletand the mixed water inlet. This may be an additional cleaning chamberwith a different physical effect with respect to the cleaning chamber.

The flow through the primary circuit in the sense of an optimaloverflowing of the membrane 49 is here ensured, namely substantiallyindependently of the action of the pump used for mixed-water supply,pressure build-up and circulation performance.

To remove substance residues, another inventive feature is that theliquid of the primary circulation circuit is conveyed by way of atangential inlet through a cylindrical centrifugal chamber on the upperend of which a turbine blade, which is rotatable by liquid pressure,conveys the substances and particles to be separated downwards andpasses the cleaned liquid through a hollow shaft or a strainer-likecylinder upwards.

A collection chamber for the particles or lime clusters to beprecipitated is located under the centrifugal chamber. The dischargevalve may be secured to the collection chamber if a centrifugal chamberis present. The centrifugal chamber may here also be arranged upstreamof the primary circuit for instance in front of the supply tank (5).

With great advantage it is also suggested that the power output stagesfor the control of the cleaning chamber should be configured such thatthey are adjustable with respect to frequency and current and should becontrolled and monitored for malfunction by the processor of the reverseosmosis system. A specific bit pattern can here be output as a testsignal and monitored by means of a watch dog. The respective operatingstatus and also the signal shape are here displayed via the displaydevice of the reverse osmosis system and stored with memory modules.

The data can be requested at any time by means of an interface e.g.Ethernet connection of the microcontroller of the reverse osmosissystem.

FIG. 2 shows the module (10) with the mixed-water inlet (9), theconcentrate outlet (13), the permeate supply (14) and the permeatecirculation line (15). The membrane primary circuit (11) is hereoperated via the pump (7), the high-pressure throttle (16), the cleaningchamber (38) and the centrifugal chamber (40) via the pressuremaintaining valve (17) and the heater (39) in such a manner that asufficiently high transmembrane pressure is created for conveying theliquid of the primary circuit (11) into the membrane secondary circuit(12). Due to the high transmembrane pressure the liquid of the membranesecondary circuit (12) (permeate) is passed over the temperature sensors(31/33), the heater (32), the conductivity cell (34) and the valve (35)via the line (42) to the consumer (44). The pump (27) must here beoperated in a selective way.

In the unconnected state of the consumer (44) the liquid can be returnedeither via the lines (23/24) or, in the connected state of the valve,via the permeate-ring safety valve (36) to the supply tank (5).

For cleaning the high-purity secondary circuit (12/23/24/48) the pump(27) is circulating the liquid via the heater (32) during thermalcleaning until a microbiologically inactivating temperature has beenreached.

When the secondary circuit is cleaned by means of the electrolytic ozonecell (28), said cell is switched on via a safety circuit and theozone/liquid mixture is circulated in a circuit by means of pump (27)for such a long time until a microbiologically inactivating effect hasbeen achieved.

The primary circuit (11/9/18) can here be deactivated with the pump (7)so that a purification of the high-purity circuit (12/23/24) isexclusively carried out.

Position and mode of operation of the pump (27) and of the ozone cell(28) are not limited to the method described.

In the event that an integrated disinfecting operation, i.e. withcoupled and operated consumer (44), is carried out, the consumer can besupplied with hot water or ozone via the opened valve (43). Adisinfection of the line (42) is also possible via the opened valve (45)without operation of the consumer (44) to the drainage (22). Thethrottle valve (46) reduces the volume flow. The switching of theconsumer valves (43/45) or the communication between consumer (44) andRO system (60) is here carried out via the communication interfaces ofthe RO (60) or of the consumer (44). The commands can here be given bythe RO (60) to the consumer (44) and also inversely.

Valve (29) is opened for the hot cleaning of the whole RO system (60)without consumer (44). The pump (7) feeds the liquid circuit such thatenough permeate flows via the heater (32). Valve (37) can here beopened. The pump (27) performs a circulatory supportive operation in thesecondary circuit (23/24/48). The heated permeate flows selectively viathe valves (26/35) or (36) back to the supply tank (5) until amicrobiologically inactivating status has been reached.

It is also possible to feed ozone in small amounts into the primarycircuit (9/18) to achieve a disinfecting action without oxidation of themembrane material there. In combination with the ozone input the heater(32) can be operated such that only a short-term ozonization that isgentle on the material is carried out.

Apart from the hot cleaning of the whole reverse osmosis system, it isalso possible to clean the liquid of the primary circuit permanently bymeans of the cleaning chamber (38). To this end the liquid is passedwith the pump (7) running via the cleaning chamber (38) and thecentrifugal chamber (40). Part of the circulating liquid is passed viathe concentrate flow meter (21) and the concentrate discharge valve (30)to the outlet.

To flush out substance residues, the high-pressure bypass valve (37) iscyclically opened and the substance residues contained in the primarycircuit are flushed out to the drainage (22) via the opened valve (20).

Optionally, the primary circuit is cleaned via the pump (61) and via acleaning chamber (38) which is either arranged in addition or isoperated alone.

With the methods described, both primary and secondary circuits of thereverse osmosis system and of the distributor circuit can be cleanedwith different methods and independently of one another in a cost-savingand efficient manner.

FIG. 2a shows a method in which the permeate collection tube 48 can becleaned separated from the permeate circuit 23/24.

To this end the high-purity liquid of the membrane secondary side 12 ispassed by means of the pump 27 via the circulation valve 66 and thepermeate collection tube circulation line 68 over the cleaning devices32 and/or 28 for such a long time until a microbiologically inactivatingresult has been achieved.

With this method the permeate circuit 23/24 or the permeate collectioncircuit 48/23/68 can be cleaned separately or together in a selectiveway.

The bypass valve 67 serves to discharge flawed permeate in the closedstate of the permeate ring shut-off valve 35.

FIG. 3a schematically shows a module (10) with inserted membrane module(63). The liquid is guided via the mixed-water connection (9) to themembrane primary circuit (11) and passes via the filter membrane (49) tothe membrane secondary circuit (12). The permeate collection tube (48)is perforated (65) in such a manner that the liquid in the membraneprimary circuit (11) which is supplied via the spirally arrangedmembrane pockets of the filter membrane (49) can enter into the permeatecollection tube. It passes from there via the connections (14/15) intothe high-purity distributor circuit. The concentrate is passed on viathe connection (13).

By comparison, FIG. 3b shows a 3-pole membrane according to the priorart. The liquid is here also supplied via the connection (9). Thepassage via the filter membrane (48) takes place as has already beendescribed under 3 a. Schematically shown is here the often employedtechnique of the series connection by means of the permeate collectiontube connector (52) and the closure of the permeate side (51). Thepermeate collection tube can here be cleaned exclusivelytransmembranically.

FIG. 4 shows the structure of a cleaning cell (38) with 3 electrodes,the middle electrode (54) being isolated spatially and electrically fromthe two outer electrodes (53). The liquid can here be introducedbidirectionally via the flow channel (55) into the cell. Due to thelarge surface distribution of the outer electrode (53) a uniformpotential distribution is achieved in the inner electrode chamber. Theisolating piece (56) serves as installation space for the middleelectrode (54). The cup-shaped outer electrodes (53) have to be equippedwith different connectors (57) such as e.g. clamp connection, plugnipple connection or hose connection.

The middle electrode (54) is inserted as an annular electrode body inthe isolating piece (56).

Depending on the application, the material of the outer electrodes (53)consists of special steel, titanium, mixed titanium oxide or sinteredcarbon.

The middle electrode (54) consists of an oxidation-stable material suchas e.g. conductive carbon, mixed titanium oxide, a ceramic mixture ofmetal oxides, titanium oxide or cobalt.

It is possible through the selection of the material and the electricalconnection type to operate the cleaning chamber (38) as an electrolysiscell or as an electromagnetic cell or as a cell with electrodeconnections for current and voltage—also capacitively.

Preferably, a pole of the electrical supply is connected to the bridgedouter electrodes (53), and the other pole to the middle electrode (54).

During operation of the cleaning chamber (38) as an electrolysis cellthe two outer electrodes (53) are the cathodes, and the middle electrode(54) is the anode.

This electrolysis cell serves to produce oxygen radicals for theinactivation of the microorganisms or also to reduce lime scale.

FIG. 4 shows the design of a combined cleaning chamber (38) with 3electrodes and a coil winding (58).

Decalcification is here carried out via the force lines of thecoil-generated magnetic field in the liquid.

The use of Teflon-encapsulated ring magnets in the liquid or ringmagnets outside the isolating piece (56) instead of the coil winding(58) is possible.

1. Feeding line 2. Measurement: input conductivity 3. Measurement:inflow 4. Water inlet valve 5. Supply tank 6. Level control 7. Pump 8.RS valve 9. Mixed water inflow 10. Filter module 11. Membrane primarycircuit 12. Membrane secondary circuit 13. Concentrate outlet (line) 14.Permeate supply 15. Permeate circulation and bypass line 16.High-pressure throttle 17. Pressure-maintaining valve 18. Concentrateoverflow 19. Concentrate discharge line 20. Concentrate discharge valve21. Concentrate flow meter 22. Drainage 23. Permeate feed line (can alsobe introduced into the consumer) 24. Permeate return line (can also beintroduced into the consumer) 25. Connection point: consumer 26.Pressure maintaining valve - permeate return 27. Circulation pump 28.Ozone cell 29. Permeate ring - outflow valve 30. UV lamp 31. Temperaturesensor 32. Heater 33. Temperature sensor 34. Conductivity cell 35.Permeate ring shut-off valve 36. Permeate ring safety valve 37.High-pressure throttle - bypass valve 38. Cleaning chamber 39. Heater -secondary side 40. Centrifugal chamber 41. Permeate ring - overflowvalve 42. Interface line 43. Consumer - input valve 44. Consumer 45.Interface line - flushing valve 46. Throttle 47. Pressure pipe 48.Permeate collection tube 49. Filter membrane 50. Membrane seal 51.Plug - permeate collection tube 52. Connector - permeate collection tube53. Outer electrodes 54. Middle electrode 55. Supplied liquid 56.Isolating piece 57. Connectors 58. Coil winding 59. RS valve 60. ROsystem 61. Circulation pump 62. RS valve 63. Membrane module 64.Cleaning sensor 65. Perforation - permeate collection tube 66.Circulation valve 67. Bypass valve 68. Permeate collector circulationline

The invention claimed is:
 1. A reverse osmosis system which can becoupled with at least one dialysis device to supply said dialysis devicewith high-purity permeate, comprising: a raw-water inlet line whichsupplies raw water through a valve to a buffer vessel with a levelcontroller and then via a pump to a filter module having a primary spacebeing separated by a semipermeable membrane from a secondary space,wherein the secondary space of the filter module is provided by aperforated permeate collection tube having first and second ends, thepermeate collection tube being arranged within spirally arrangedmembrane pockets, the secondary space being connected to a permeatedistribution system comprising a heater for cleaning and/or disinfectingpermeate, a permeate inlet line with at least one connection to whichthe dialysis device can be coupled, a permeate return line downstream ofthe permeate inlet line into which a permeate outlet valve is insertedthrough which, in an open state, the permeate can flow in a firstdirection through the permeate return line and then out of the permeatereturn line via the permeate outlet valve into the buffer vessel, and ina closed state, the permeate flowing through the permeate return line inthe first direction bypasses the buffer vessel and the permeate outletvalve and then returns to the second end of the secondary space withoutfirst passing through the primary space or the permeate inlet line, thepermeate distribution system further comprising a circulation pumpdisposed downstream of the permeate outlet valve and upstream of thesecond end of the secondary space, said circulation pump being inaddition to said pump; and a concentrate outlet line leading away fromthe primary space of the filter module to the buffer vessel, wherein atleast the heater, the permeate inlet line, the permeate return line, andthe circulation pump are arranged in a circulation circuit in which thepermeate flows in the first direction, wherein the perforated permeatecollection tube is connected at both the first and second ends to thecirculation circuit, the first end being connected to the permeate inletline and the second end being connected to the permeate return line suchthat the circulation circuit is formed in which the permeate cancirculate.
 2. The reverse osmosis system according to claim 1, whereinthe circulation circuit branches off from the permeate inlet line andencloses a section thereof.
 3. The reverse osmosis system according toclaim 1, wherein the permeate distribution system has inserted thereintoa check valve which is operative in such a manner that the permeate canflow from the filter module through the permeate inlet line, but notthrough the permeate return line.
 4. The reverse osmosis systemaccording to claim 3, wherein the check valve is arranged in thepermeate return line in the vicinity of the first end of the permeatecollection tube.
 5. The reverse osmosis system according to claim 1,wherein the heater is connected to a temperature measuring device. 6.The reverse osmosis system according to claim 1, wherein the permeatedistribution system includes at least one ozone cell.
 7. The reverseosmosis system according to claim 1, wherein a pressure maintainingvalve shuts off a return of the permeate to the at least one connection,and is inserted into the permeate return line between the junction of abypass line and the at least one connection.
 8. The reverse osmosissystem according to claim 1, wherein the raw-water inlet line isconnected via a raw-water bypass line to the concentrate outlet line,and a circulation pump and a cleaning chamber for the raw water areinserted into the raw-water bypass line.
 9. The reverse osmosis systemaccording to claim 8, wherein the cleaning chamber is provided withthree electrodes and serves to produce oxygen radicals for theinactivation of microorganisms and to reduce lime deposits.
 10. Thereverse osmosis system according to claim 1, wherein the concentrateoutlet line communicates with a drainage, and wherein a centrifugalchamber is arranged in flow direction upstream of the drainage.
 11. Thereverse osmosis system according to claim 10, wherein the centrifugalchamber has an optical sensor which detects the degree of soiling. 12.The reverse osmosis system according to claim 1, further comprising: aconductivity cell and a permeate safety valve, both the conductivitycell and the permeate safety valve being arranged in the circulationcircuit, the permeate safety valve connecting both the permeate inletline and the permeate return line, and forming a bypass line bypassingthe at least one connection to which the dialysis device can be coupled;a shut-off valve arranged in the permeate inlet line downstream of aconnection of the permeate safety valve to the permeate inlet line andupstream of the at least one connection to which the dialysis device canbe coupled, and the conductivity cell being arranged in the circulationcircuit on or upstream of the connection of the permeate safety valve tothe permeate inlet line; and a pressure maintaining valve disposedbetween the dialysis device and a junction of the bypass line connectingwith the permeate return line.